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Full text of "Handbook Of Chemical Engineering - I"

418                               CHEMICAL ENGINEERING
TABLE 6.—CHARACTEKISTICS or PRESSURE THERMOMETERS
Designation	Material available for filling	Approximate temperature range, degrees Centigrade	Scale
Vapor pressure ..........	Ethyl ether Ethyl alcohol	40 to 190 80 to 240	Unevenly divided Unevenly divided
Liquid filled ...... . .......	Benzene Aniline J Alcohol	80 to 280 200 to 425 up to 200	Unevenly divided Unevenly divided Evenly divided
Gas filled ...............	{ Mercury Nitrogen	up to 550 up to 425	Evenly divided Evenly divided
			
Liquid
This is a decided advantage over other types, if the connecting tubing and gage are both to be subjected to considerable changes in temperature. On the other hand, if. the vapor-pressure thermometer is not filled properly, i.e., the proportion of liquid is too great or too small as compared with the volume of the bulb, capillary and gage, large and uncertain errors may be introduced. As an example take the condition met
Gage with in the use of high-temperature vapor-pressure thermometers. The bulb is usually much hotter than the capillary which is filled or nearly filled with vapor so that the liquid is condensed in the capillary and there should be sufficient liquid in the system to entirely fill the capillary and gage and partly fill the bulb. If the liquid only partly fills the capillary, and there is no liquid in the bulb, the capillary will contain the free surface of the liquid and the temperature indicated will be that of the portion of the capillary containing the free surface of the liquid as shown in Fig. 4. This temperature, which may be several hundred degrees lower than that of the bulb, will be indicated instead of the true FIG. 4,—Vapor-pressure thermometer.          temperature of the bulb.
Gas- and liquid-filled thermometers operate on the principle of thermal expansion. They are entirely filled with either the liquid or the gas. The expansion of the liquid or gas in the bulb is transmitted through capillary tubing to the pressure gage. These instruments are subject to error if the gage and capillary are heated or cooled to temperatures differing from those under which they were calibrated. This error may be made negligible in many instances by reducing the volume of the capillary and gage as compared with that of the bulb, or by using a compensator in the head. Such a compensator, however, will not eliminate errors due to the heating or cooling of the capillary alone.
On account of the mechanical construction of pressure thermometers, both as regards the mechanism for indicating and recording and the necessity for using comparatively large and heavy bulbs, the accuracy that can be secured with this type
Free surface of liquid instrument will indicate temperature of this point
Vapor